The present invention relates to an apparatus for generating surface pressure, in particular in an injection molding machine, having a stationary carrier platen and a clamping platen which can be moved in relation to said carrier platen and locked in a working position and which, on its side facing the carrier platen, has a closing platen that can be moved electromechanically. The material to be pressed or tools to be clamped are disposed between said closure platen and the carrier platen.
Generating a surface pressure as described above is generally known from the reference DE 43 36 572 C1. As disclosed in this reference, the clamping platen and closing platen of an injection molding machine are moved relatively quickly by a first moving mechanism, and then the clamping platen is locked in the working position. The injection molding operation is started and, in order to cushion the forces originating from the plastics flow front, the closing platen is moved out by an alternating magnetic field being applied to a positively magnetostrictive material.
However, an electromechanical movement acting on this principle has the disadvantage that relatively high current intensities are needed. In addition, a permanent current has to be maintained, even in the stationary state.
It is an object of the invention to construct an apparatus for generating surface pressure of the type mentioned above in such a way that a rapid and powerful electromechanical movement is achieved in the technically simplest possible way.
According to the present invention, the above-stated object is achieved by the compressive force of the closing platen, with the action of moving the closing platen at the working point being implemented by piezoelectric actuators. While these components have been used for a long time for various tasks in engineering, neither have been proposed nor used for moving closing platens in apparatus for generating surface pressure.
One embodiment of the invention is characterized by the fact that the piezoelectric actuators are distributed, preferably in matrix fashion, over the area between clamping platen and closing platen. This results in an arrangement of the actuators which can be implemented simply in technically comprehensible terms. However, it is also possible for the piezoelectric actuators to be distributed over the area between clamping platen and closing platen in accordance with the desired force distribution i.e., hardware implementation of the force distribution. The manner in which such an arrangement is to be made can be determined analytically or by means of trials. A second possibility, corresponding with a force distribution, consists in actuating the piezoelectric actuators differently i.e., software implementation of the force distribution.
In the event that the behavior of the material to be pressed or the tools to be clamped exhibits a dynamic behavior, the piezoelectric actuators can be triggered dynamically, to match said behavior, such that the responsive behavior can be configured so as not to damage the machine.
For the case in which relatively long distances have to be bridged with the aid of the piezoelectric actuators, it proves to be advantageous if the closing platen can also be locked in at least one intermediate position which makes up a piezoelectric stroke and from which the clamping platen can subsequently be guided, the latter then being locked and the closing platen being moved out by a further piezoelectric stroke.
The piezoelectric actuators can be produced with any desired geometry in accordance with production possibilities, and can therefore be matched relatively easily to all machine requirements. A cube-like geometry is preferred.
Additional piezoelectric sensors may be provided between closing platen clamping platen, so that pressure measurements can be performed completely independently of the piezoelectric actuators. However, it is also possible that, during operation, a subset of the piezoelectric actuators can be used as piezoelectric sensors and, likewise, it is also possible that, during operation, piezoelectric actuators can be used briefly as piezoelectric sensors. The foregoing alternatives have the advantage that double use of the piezoelectric actuators is made, with the latter being used as piezoelectric sensors.